Output stages of AM-RF transmitters are supplied with DC power supply voltages, typically derived from AC to DC converters. High power transmitters, i.e., transmitters with at least a 5 kilowatt output, are usually driven by three-phase AC power supply sources derived from AC mains or an AC generator.
It is known that variations in the DC supply voltage supplied to an AM-RF transmitter output stage cause distortion in a transmitted AM-RF carrier whereby modulation imposed on the carrier by an information signal prevents accurate reception and recovery of the information signal. The information signal is typically in the audio range and frequently constitutes voice or music signals. The distortion occurs because the power supply variations change the amplitude of the transmitted carrier wave envelope.
To avoid distortion causing variations in the DC power supply voltage supplied to an output stage of an AM-RF transmitter, there AC to DC converters are designed in such a way as to minimize ripple and other changes of the DC power supply voltage. Typically, the prior art has employed two different types of AC to DC converters to provide the stabilized DC power supply voltage.
One type of prior art AC to DC converter includes a three-phase transformer and associated full wave rectifiers which drive a low pass filter including a series inductor and shunt capacitor. In a second arrangement, that produces a DC voltage having less ripple than the first type of AC to DC converter, a three-phase to twelve-phase transformer arrangement is provided. Each of the twelve phases drives a full wave rectifier. DC voltages derived by the twelve different full wave rectifiers are supplied to a single, shunt capacitor having a very large capacitance.
Both prior art converters have resonant frequencies which we have found introduce distortion in the transmitted AM-RF carrier envelope. The resonant frequencies of the converters including the series inductor and shunt capacitor are in the 50 Hertz range. The resonance of these converters is caused by the reactances of the discrete, lumped parameter inductor and capacitor forming the low pass filter. The resonant frequency of the AC to DC converters including the twelve-phase transformers is considerably higher, approximately 100 Hertz. Resonance of the converters including the twelve-phase transformer is due to leakage and distributed inductance and capacitance of the transformer, in combination with the capacitance of the shunt, filter capacitor.
We have found that the reactances of both types of AC to DC converters interact with the information signal when the current demands of the transmitter output stage are greatest, i.e., at the time that the modulation imposed on the output stage has the greatest amplitude, and when the information signal frequency approaches or is approximately the same as the resonant frequency of the converter. (For the purposes of this specification and the claims appended hereto, the information signal frequency, f.sub.1, is considered to approach the resonant frequency, f.sub.0, when f.sub.1 is equal to or less than 3f.sub.0 and when 3f.sub.1 is equal to or less than f.sub.0. ).
Even though the lumped parameter filters have a resonant frequency below the lowest audio frequency that modulates the transmitter, the distortion occurs because varying current demand occurs at syllabic frequencies, which are typically in the 1/2 to 25 Hertz range, i.e., in a frequency range approaching the resonant frequencies of the lumped parameter filters. In addition, when low audio frequencies of the information signal approach the lumped parameter filter resonant frequency, amplitude and phase distortion in the transmitted carrier occur because the filter has a high enough impedance to cause a downward fluctuation of the power supply voltage during the positive half cycle of the low frequency modulation imposed on the transmitter output stage.
The resonant frequency of the AC to DC converters including the twelve-phase transformer is in the same range as the lowest frequencies (typically about 100 Hz) of the audio modulating, information signal. Frequently, some resistive element is provided in the converter to broaden or flatten the resonant effect to avoid the distortion resulting from an interaction of the low audio frequencies of the modulating source and the power supply. In this situation, the degree of regulation of the AC to DC converter determines how much DC voltage fluctuation occurs with varying current demands as a function of modulation. With low power transmitters, AC to DC converters with sufficiently low impedance may be available. However, with higher powered transmitters, i.e., power ratings of at least 5 kilowatts, there are greater load variations making it quite difficult to provide a suitably stiff source for supplying the transmitter DC energizing voltage.
Hence, with transmitters employing both of the stated types of AC to DC converters, it is desirable to reduce distortion resulting from variations in the DC power supply voltage applied to an AM-RF transmitter output stage during modulation by low frequencies in the audio range.
It is, accordingly, an object of the present invention to provide an AM-RF transmitter having reduced carrier wave distortion in response to low audio modulating frequencies.
It is another object of the present invention to provide a new and improved AM-RF transmitter with distortion compensation resulting from variations in DC power supply voltage applied to the transmitter output stage due to an interaction between low frequencies of an audio modulating source and reactances of an AC to DC converter having a resonant frequency at least approaching frequencies of the modulating, information source for the transmitter.
An additional object of the invention is to provide a new and improved AM-RF transmitter with distortion compensation resulting from variations in DC energizing voltage applied to the transmitter output stage, wherein the compensation is provided with an inexpensive, light weight and small circuit.